Method for producing an electromagnetic steel sheet of a thin sheet thickness having a high-magnetic induction

ABSTRACT

A process for producing a very thin electromagnetic steel sheet having a high-magnetic induction by the steps of hot-rolling a steel ingot containing less than 0.085 percent C, 0 to 4 percent Si, 0.003 to 0.100 percent S and 0.010 to 0.065 percent acidsoluble A1, subjecting the hot-rolled steel sheet to an intermediate annealing and further to cold-rollings of two times.

United States Patent 1111 3,632,456

[ Inventors T M Y m Sam Wm [56] Relerencel Cited ammo ru Kiyoshl Ueno, all of Kitakyusllu Japan STATES PATENTS [2 App. 819,426 3,069,299 12/1962 Fredler 148/1 1 l [22] Filed Apr. 25 969 3,147,158 9/1964 Fredler 148/1 1 1 ml Madame Wm SM comnmn 3,159,511 12/1964 Taguchi et al. 148/111 Tokyo, hp" 3,184,346 5/1965 Fiedier 148/1 1 l X [32] Priority Apr. 27 1968 3,214,303 10/1965 Fledler I48]! 1 1 [33] Jam 3,266,955 8/1966 Taguchi et al. 148/1 1 l [3 43/283 3,287,183 11/1966 Taguchi et al. 148/111 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White [54] METHOD FOR PRODUCING AN Attorney-Wenderoth, Lind 8L Ponak ELECTROMAGNETIC STEEL SHEET OF A THIN SHEET THICKNESS HAVING A HIGH-MAGNETIC INDUCTION ABSTRACT; A process for producing a very thin electromag- 4 Claims, No Drawings netic steel sheet having a high-magnetic induction by the steps of hot-rolling a steel ingot containing less than 0.085 percent [52] n 3 7 C, 0 to 4 percent Si, 0.003 to 0.100 percent S and 0.010 to [51] Int m on 1/16 0.065 percent acid-soluble Al, subjecting the hot-rolled steel 501 mm or Search 148/1 10, imemediale meal'ng and 1 12 oftwotimes.

METHOD FOR PRODUCING AN ELECTROMAGNETIC STEEL SHEET OF A THIN SHEET THICKNESS HAVING A HIGH-MAGNETIC INDUCTION This invention relates to a method for producing singleoriented electromagnetic steel sheets having an easy magnetization axis 100 in the rolling direction of the steel sheet.

Single-oriented steel sheets as soft magnetic materials are to be used mostly as iron cores for transformers and other electric devices. For their magnetic characteristics, excitation characteristics and core loss values must be favorable.

This present inventors use the magnetic induction B (gausses) generated in the iron core by the intensity H (Oersted) of the magnetic field as a value for expressing the excitation characteristics, and the core loss W /50 (W/kg.) in the case of 50 cycles and an alternating current magnetic induction of l5,000 guasses as a value for expressing a core loss, that is, an energy loss lost from the iron core in case a prescribed alternating current magnetic induction is given to the iron core.

Recently, it has become an important problem to make the size of electric devices such as transformers small and for it the reduction in the weight of the core is required. In general, in order to be able to reduce the weight of the core to be used for electric devices a portion of a steel sheet, wherein the magnetic induction is high, must be used. Thus, a steel sheet having a favorable magnetization characteristics, that is, a steel sheet high in 8 characteristics, is needed, particularly a greater importance is attached to a steel sheet having a high saturated magnetic induction 8,. Further, it is to note that when using that portion of steel sheet, wherein the magnetic induction is high, the core loss value generally increases however, as compared with a material low in B characteristics a steel sheet high in B characteristics shows a much lower core loss in the region of high-magnetic induction, and moreover the increase in the core loss is lower in its rate, as the magnetic induction rises. Further, as a countermeasure for improving the core loss itself it became necessary to obtain products having a thin gauge thereby to reduce an eddy current which accounts for a great proportion of the core loss.

In general, in case a single-oriented silicon steel sheet is used as a iron core for electric devices, products having 0.5 to 0.30 mm. thickness became the object to be obtained. However, recently products having such a thin gauge as 11 mils, 9 mils, or even 6 mils or 4 mils are demanded for use not only in special high-frequency devices, but also in iron cores for general electric devices. But, the production of products of such a thin gauge as above mentioned could not sufficiently be performed only by conventional techniques concerned with the so-called two-stage cold-rolling method formerly adopted. Consequently, a new development had to be expected by utilizing a technique like a sulfur-cementation or selenium-cementation, as is disclosed in US. Pat. No. 3,333,111. At present a single-oriented silicon steel sheet having a thin gauge is being high-frequency by the method, for instance, according to U.S. pat. No. 2,473,156.

The present invention has for its object the provision of products which can meet the demand on the market as above mentioned, the sheet thickness of which is maximum 0.35 mm. (14 mils), more particularly the provision of the method for producing a very thin single-oriented electromagnetic steel sheet, the thickness of which is centering around about 6 mils, immediately from a hot-rolled steel sheet by the multistage cold-rolling method without need of utilizing the S- or Se-cementation method. The products of the present invention are particularly distinguished by an excellent B characteristic, which amounts to at least 18,500 gausses and maximum 20,100 gausses.

In the method according to the present invention which relates to a method for producing a very thin single-oriented electromagnetic steel sheet having a high-magnetic induction, in which a normal steel, material or silicon steel material which contains C and Al are indispensable elements is produced according to known steelmaking methods, melting methods and casting methods, which are applied usually as normal industrial techniques, the thus obtained steel material is subjected to a continuous annealing at 750 to l ,200 C. and then to cold-rollings of two times, interposing an intermediate annealing between them, thereby to obtain a desired final gauge, the thus obtained product is decarburized and then subjected to a final annealing to produce secondary recrystalli zation grains of{l l0} l 00 -orientation in the steel material, the present invention is particularly characterized in that the annealing of the hot-rolled steel sheet is carried out in a hightemperature range of 750 to l,200 C. to cause AlN' to precipitate in desirable size, whereby after the final annealing the magnetic induction B in the rolling direction may reach such a high level as at least [8,500 gausses and maximum 20,100 gausses.

An object of the present invention is to provide a method for producing a very thin electromagnetic steel sheet having a high-magnetic induction.

Another object of the present invention is to provide a chemical composition suitable for producing a very thin electromagnetic steel sheet having a highmagnetic induction.

The details of the present invention shall be explained in the following.

The normal steel or silicon steel material which is a starting material in the present invention means an ingotrnade by solidifying by any casting method a molten steel madeby such a steelmaking method which is already known art, as, for example, by open-hearth furnace, electric furnace or converter or melted by such a known melting method as, for example, by a high-frequency electric furnace or vacuum melting furnace. A slablike ingot obtained by a continuous casting method, which recently came into wide use, can also be used as a material in the present invention. The atmosphere in the case of casting is usually of air but may be vacuum or of an inert as well.

As described above, the material in the present invention may be made by any steelmaking melting and casting methods. But the composition of the material must satisfy the following conditions, irrespective of the methods for producing the same, that is, steelmaking melting and casting methods. That is, the steel ingot must contain C less than 0.085%

Si O to 40% Al 0.010 to 0.065% (Al means an acid-soluble Al, which will be hereinafter abbreviated simply as Al) S 0.003 to 0.l00%

as indispensable elements, and the rest means Fe and other elements which are usually to be contained inevitably in an electromagnetic steel sheet of this kind. C must be added depending upon the Si content, butin an amount sufficient to effect a 'y-transformation at least at a part of the steel material. Al is an absolutely necessary element contributing in the form of Al nitride to the production of secondary recrystal grains of a particularly excellent orientation at the time of the finalannealing. Further, S acts in the form of a sulfide to inhibit the normal grain growth which are not good in the orientationat the time of the final annealing, stably produces secondary recrystal grains having a very sharply regulated orientation ,by its coexistence with Al nitride and has made it possible to industrially produce single-oriented thin electromagnetic steel sheets having a high-magnetic induction. In the present invention, when Al is 0.010 to 0.065 percent and S is 0.003 to 0. percent, the above-mentioned object can be attained. In annealing a hot-rolled sheet, C is adjusted to be below 0.080 percent in consideration of the Si content so that a y-transformation may occur at least at a part of the steel sheet. That is to say, the summary is shown to be that, when Si is 0 to 1 percent, C should be less than 0.080 percent (ingot C being less than 0.085 percent), when Si is l to 2.5 percent, Cshould. be 0.010 to 0.080 percent (ingotC being 0.0l5 to 0.085 percent) and, when Si is 2.5 to 4.0 percent, C should be 0.020 to 0.080 percent (ingot C being 0.025 to 0.085 percent). 1t is because the amount of decarburization in the hot-rolling is taken into consideration that C in the ingot is 0.005 percent higher than that in the hot-rolled sheet before the annealing. Essentially, in annealing the hot-rolled sheet, the C content in the steel sheet is important. When the C content is above the above-mentioned range, the production of secondary recrystal grains will be imperfect and, even in case secondary recrystal grains are obtained, the degree of accumulation of the secondary recrystal grains will be low the the objective product of the present invention will not be able to be obtained. Therefore, the upper limit of the C content in the present invention is made 0.085 percent. Si is made to 4 percent. As the present invention has it as an object to improve B characteristics and B,, the lower limit of Si is not defined. But, if 4 percent is exceeded, the industrial cold-rolling will become impossible. Therefore, the upper limit is made 4 percent.

It is Al nitride that is the base of the production of secondary recrystal grains having a particularly excellent orientation in the present invention. Therefore, the presence of any other nitride-producing element than Al in relation to the production of this AlN must be noted in the present invention. The presence of the nitride contributes to secondary recrystals in the sense of inhibiting the normal grain growth and the presence of the nitride-producing element is significant but this is to be determined in relation to AlN. That is to say, such elements stronger in the affinity with N than Al as Zr and Ti may be added by taking it into consideration that AlN can be deposited by a fixed amount after the later described annealing in comparison with N percent contained in the steel. B, Ta, Nb, V, Cr, Mn, W and Mo are weaker in the affinity with N than Al and therefore may well be added by proper amounts already known in the production of single-oriented silicon steel sheets. The allowable maximum values of the amounts of addition of these elements are shown to be 1 percent of each of V, Mn and M0, 0.5 percent W and 0.1 percent of each of B, Zr, Ti, Nb, Ta and Cr. Needless to say, proper amounts of Se and Te may be contained. However, all of these are only examples. It is not deviating from the spirit of the present invention to add elements for the purpose of producing deposits for the acceleration of the production of secondary recrystals within a range not obstructing the production of AlN which is the base of the present invention. A high-magnetic induction single-oriented thin electromagnetic steel sheet which is very good in the orientation is produced by making steel ingot conforming to the above-mentioned composition prescription a hot-rolled sheet, annealing it at a high temperature for a short time, then cold-rolling it with intermediate annealing so as to be of a desired product gauge of a thickness of less than 0.35 mm. or mostly about 6 mils, then decarburizing it and then finally annealing it. The features of the present invention are as follows. That is to say, the high-temperature annealing of the hot-rolled sheet is to precipitate AlN of a desirable size and is carried out by a continuous annealing wherein the steel sheet is held in a temperature range of 750 to 1,200 C. for 30 seconds to 10 minutes and is then cooled at a comparatively high speed or preferably quenched. The cooling speed after the steel sheet is held at a required temperature for a required time in the continuous annealing is such that it is preferably quenched by forced cooling within a time less than 200 seconds from 750 to 400 C. when Si is less than 1 percent, from 850 to 400 C. when Si is 1.0 to 2.5 percent or from 950 to 400 C. when Si is 2.5 to 4 percent. The annealing after the hot-rolling has been carried out mostly in the form of a box annealing or open annealing. However, not only in the box annealing but also in the open annealing, it has been thought that quick cooling has such bad influences as giving an unfavorable stress to the steel sheet and deteriorating its shape and that therefore, unless it is not a remarkable loss to the economy, the steel sheet should be gradually cooled.

1n the case of the present invention, AlN acts to give an excellentorientation to a steel sheet. However, such action is greatlyinfluenced by the size of the precipitated AlN. Therefore, in order to precipitate a desirable size of AlN-, various means have been attempted. The quenching after the abovedescribed continuous annealing is one of means effective to improve the magnetic characteristics of the product and thereby a single-oriented thin silicon steel sheet of a B characteristic of at least 18,500 gausses can be produced. 1f the cooling speed is lower than in the above-mentioned conditions, that is to say, in the conventional system, it is difficult for the B characteristic to exceed 18,500 1 gausses. In the publication of 11.8. Pat. No. 3,287,183 is mentioned a process for producing a single-oriented silicon steel sheet of a highmagnetic induction in which the B characteristic of the final product is 18,000 to 19,100 gausses in a step including an annealing at a high temperature for a short time before a final strong cold-rolling and the final strong cold-rolling by treating a silicon steel ingot containing Al. However, in the present invention, it has been able to be made possible to produce a thin high-magnetic induction single-oriented silicon steel sheet in which the B characteristic of the final product reaches at least l8,500 gausses or a maximum of 20,100 gausses by treating it in a two-step cold-rolling process including a final strong cold-rolling after holding it at such high temperature as is described above after hot-rolling and continuously annealing it by cooling it at a comparatively high speed.

The cooling speed from the annealing holding temperature range of 750 to l,200 C. to the temperature range of 750 to 950 C. corresponding to the Si content and the cooling speed below 400 C. are not particularly specified. The amount of AlN required to attain the object of the present invention may be at least 0.0005 percent (N as AlN) after the hot-rolled sheet is continuously annealed at a high temperature. Therefore, the atmosphere for annealing such hot-rolled sheet may be anything proper for the precipitation of such AIN, Usually a steelingot obtained in an open hearth furnace contains more than 0.0040 percent N as it is and this is an amount sufficient to precipitate required AlN. Therefore, unless a remarkable denitrification occurs in the atmosphere, the atmosphere may well be such reductive or neutral atmosphere as, for example, of H Ar or N or a gaseous mixture of them or of air. However, in case the steel ingot has been obtained by vacuum dissolution or the like, N will be so extremely little that it will be necessary for the annealing atmosphere to contain N,.

In the present invention, the cold-rolling step is carried out twice and the last cold-rolling is a strong cold-rolling which must be carried out at a reduction rate of 70 to percent. With a reduction rate of less than 70 percent, no product of a high B characteristic will be obtained. The reduction rate in the first cold-rolling need not be particularly specified but is properly selected to be 5 to 40 percent by taking the thickness of the hot-rolled sheet and the thickness of the product into consideration. In the present invention, as a thin product is an object, as a result of investigations in relation to the reduction rate in the strong cold-rolling, the thickness of the hot-rolled sheet may be less than 5 mm. or preferably 2.6 to 3.4 mm. The intermediate annealing to be carried out between the two cold-rollings may be an annealing with a time and temperature sufficient to make the cold-rolled structure a primary recrystal structure.

As shown in the present invention, in the case of obtaining a thin high-magnetic induction single-oriented silicon steel sheet of a thickness less than 0.35 mm., if a hot-rolled sheet is continuously annealed at a high temperature and a high cooling speed and is then treated in a two-step cold'rolling process including a final strong cold-rolling step, not only a great effect will be recognized in the improvement of the characteristics but also the conditions of the composition and treating steps will be greatly reduced and thus it has become possible to produce a thin high-magnetic induction single-oriented silicon steel sheet of a thickness of less than 7 mils which has been quite impossible before.

For the decarburizing annealing of the final gauge steel sheet may be used any known method. The final annealing is carried out at such temperature for such time that secondary recrystal grains in the{110} 100 orientation may sufficiently develop. In order to develop secondary recrystal grains, it is desirable to carry out the final annealing in a temperature range in which no 7 is produced in response to the Si content and at a temperature as high as possible. When Si is less than 1 percent, such temperature should be 950 C. at most and usually lower. When the Si content is more than 2 percent, a

high temperature above 1,000 C. is possible. On the other hand, below 800 C., no secondary recrystal will occur. The annealing time is sufficient with more than 1 hour for the production of secondary recrystal grains but is more than 5 hours in order to obtain a product of a low iron loss value with high Si. Further, even if the atmosphere is neutral, reductive or weakly oxidative, the objective product of the present invention can be obtained.

In the above, there has been explained mostly a method of annealing a hot-rolled sheet to precipitate AlN. Further, according to the finding by the present inventors, when the above-mentioned AlN precipitating annealing is carried out again instead of the intermediate annealing between the coldrollings in the present invention, it will be effective to prevent the fluctuation of the products and will be high in the industrial effect.

Example 1 A silicon steel ingot containing 0.029 percent C, 1.02 percent Si, 0.040 percent S and 0.026 percent Al, said ingot being prepared by an electric furnace, was bloomed and hot-rolled to be a hot-rol1ed steel sheet 3.1 mm. thick. After this hotrolled steel sheet was continuously annealed in N at 1,050 C. for 1 minute, it was pickled and then cold-rolled at a reduction rate of about 40 percent to make the thickness of the sheet to 1.86 mm. The cold-rolled sheet of this intermediate gauge was continuously annealed in N at 800 C. for 1 minute and then strongly coldrolled at a reduction rate of 81.2 percent. Thereafter, the steel sheet was again subjected to a continuous decarburizing annealing in N at 800 C. for 2 minutes and subsequently to the final annealing in H at 950 C. for hours. The product (0.35 mm. thick) showed the magnetic characteristics in the rolling direction as follows:

B, ,=19,800 gausses W, =1.95 W/kg.

EXAMPLE 2 A silicon steel sheet containing 0.030 percent C, 2.15 percent Si, 0.015 percent S and 0.035 percent A], said ingot being prepared by an electric furnace, was bloomed and hot-rolled to be a hot-rolled steel sheet 3.0 mm. thick. After this hotrolled steel sheet was continuously annealed in N at 1,l00 C. for 1 minute, it was pickled and cold-rolled at a reduction rate of about 40 percent to make the thickness of the sheet to 1.80 mm. The cold-rolled steel sheet having this intermediate gauge was then continuously annealed in N at 1,000 C. for 1 minute. After pickling, the steel sheet was subjected to the final strong cold-rolling at a reduction rate of 83.3 percent to make the thickness of the sheet to 0.30 mm. and thereafter to a decarburizing and the final annealing at 1,200 C. for hours. The magnetic characteristics in the rolling direction of the product were as follows:

B, 19,740 gausses W, =1.02 W/kg.

EXAMPLE 3 A silicon steel ingot containing 0.053 percent C, 2.96 percent Si, 0.051 percent 8 and 0.032 Al, the said ingot being prepared by an open-hearth furnace, was bloomed and hotrolled to be a hot-rolled steel sheet 3.0 mm. thick. The content of C of the hot-rolled steel sheet was 0.050 percent. After this hot-rolled steel sheet was continuously annealed in N; at l,100 C. for 2 minutes, it was quenched. After pickling, the steel sheet was cold-rolled at a reduction rate of about percent to make the thickness of the sheet to an intermediate gauge of 2.1 mm. Then, the steel sheet was annealed at 800 C. for 1 minute and thereupon was finally cold-rolled at a reduction rate of 85.7 percent to make the thickness of the sheet to 0.30 mm. The steel sheet was then subjected to a continuous decarburizing annealing and to the final annealing in H at 1,150 C. for 25 hours. The magnetic characteristics in the rolling direction of the product were as follows:

B =l9,350 gausses W, =0.89 W/kg.

EXAMPLE 4 A silicon steel ingot containing 0.041 percent C, 3.10 percent Si, 0.021 percent S and 0.030 percent Al, the said steel ingot being prepared in a converter, was bloomed and hotrolled to be a hot-rolled steel sheet 2.2 mm. thick. After this hot-rolled steel sheet was continuously annealed in N at 1,120 C. for 1 minute, it was cold-rolled at a reduction rate of 70 percent to be a cold-rolled steel sheet having an intermediate gauge of 0.67 mm. This cold-rolled steel sheet was continuously annealed in N at 900 C. for 1 minute and then subjected to the final strong cold-rolling at a reduction rate of 84.7 percent to make the thickness of the sheet to 0. 102 mm. (4 mils). This steel sheet of the final gauge was decarburized at 800 C. for 1 minute, while passing through a decarburizing furnace together with a steel-made belt so that no tension may be imposed. The magnetic characteristics of the product after the final annealing (carried out at 1,200 C. for 20 hours) were as follows:

B, =18,890 gausses W, =0.6l0 W/kg.

EXAMPLE 5 A silicon steel sheet containing 0.050 percent C, 3.15 percent Si, 0.035 percent S and 0.021 percent Al, the said steel ingot being prepared in an open-hearth furnace, was bloomed and hot-rolled to be a hot-rolled steel sheet 3.0 mm. thick. After the hot-rolled steel sheet was held in a continuousannealing furnace containing N at l,l00 C. for 1 minute, it was quenched by means of a high-pressure water-spraying device installed at the port of the furnace. After pickling, the steel sheet was cold-rolled at a reduction rate of 51 percent to be a cold-rolled steel sheet having an intermediate gauge of 1.47 mm. This cold-rolled steel sheet was again annealed at 1,100 C. for 1.5 minutes and then cooled by an N -gas blowing device installed at the port of the furnace. Thereupon, the steel sheet was subjected to the final strong cold-rolled at a reduction rate of 89.6 percent to make the thickness of sheet to 0.122 mm. (6 mils) and thereupon to a continuous decarburizing. The final annealing was carried out at 1,200 C. for 25 hours. The magnetic characteristics in the rolling direction of the product were as follows:

- B, l9,120 gausses W, =0.68 W/kg.

What is claimed is:

l. A process for producing a very thin electromagnetic steel sheet having a high-magnetic induction, comprisingthesteps of blooming a steel ingot containing less than 0.085, percent C, 0 to 4 percent Si, 0.003 to 0.100 percent S and 0.010 to 0.065 percent acid-soluble Al. and hot-rolling the said ingot to make a hot-rolled steel sheet, annealing the hot-rolled steel sheet to cause AlN to precipitate in the steel sheet by maintaining the steel sheet in a temperature range of 750 to 1,000 C. for 30 seconds to 10 minutes, rapidly cooling the steel to about400 C., subjecting the annealed steel sheet to cold-rollings of at least two times, including the final cold-rolling to be-carried out at a reduction rate of 70 to percent to make the thickness of the sheet to the final gauge of 0.35 to 0.05 mm., with an intermediate annealing to be carried out between the said cold-rollings to produce the primary recrystallization structure in the steel sheet, and subjecting the steelsheet of the final gauge to a decarburizing annealing and subsequently to the final finishing annealing.

2. A process for producing a very thin electromagnetic steel sheet having a high-magnetic induction, comprising the steps of blooming a steel ingot containing less than 0.085 percent C, to 4 percent Si, 0.003 to 0.100 percent S and 0.010 to 0.065 percent acid-soluble Al and hot-rolling the said steel ingot to make a hot-rolled steel sheet having a thickness of 5 mm., annealing the hot-rolled steel sheet to cause AlN to precipitate in the steel sheet, while holding the steel sheet in a temperature range of 750 to 1,200 C. for 30 seconds to minutes, rapidly cooling the steel sheet to about 400 C., subjecting the annealed steel sheet to cold-railings of two times, wherein the first cold-rolling is carried out at a reduction rate of 5 to 40 percent and the final cold-rolling at a reduction rate of 70 to percent, to make the thickness of sheet to the final gauge of 0.35 to 0.05 mm., with an intermediate annealing to be carried out between the said cold-rollings to produce the primary recrystallization structure in the steel sheet, and subjecting the said steel sheet of the final gauge to a decarburizing annealing and subsequently to the final finishing annealing.

3. A process according to claim I, wherein the rapid cooling of the steel sheet to about 400 C. is effected by quenching the steel sheet.

4. A process according to claim 2, wherein the rapid cooling of the steel sheet to about 400 C. is effected by quenching the steel sheet. 

2. A process for producing a very thin electromagnetic steel sheet having a high-magnetic induction, comprising the steps of blooming a steel ingot containing less than 0.085 percent C, 0 to 4 percent Si, 0.003 to 0.100 percent S and 0.010 to 0.065 percent acid-soluble Al and hot-rolling the said steel ingot to make a hot-rolled steel sheet having a thickness of 5 mm., annealing the hot-rolled steel sheet to cause AlN to precipitate in the steel sheet, while holding the steel sheet in a temperature range of 750* to 1,200* C. for 30 seconds to 10 minutes, rapidly cooling the steel sheet to about 400* C., subjecting the annealed steel sheet to cold-rollings of two times, wherein the first cold-rolling is carried out at a reduction rate of 5 to 40 percent and the final cold-rolling at a reduction rate of 70 to 95 percent, to make the thickness of sheet to the final gauge of 0.35 to 0.05 mm., with an intermediate annealing to be carried out between the said cold-rollings to produce the primary recrystallization structure in the steel sheet, and subjecting the said steel sheet of the final gauge to a decarburizing annealing and subsequently to the final finishing annealing.
 3. A process according to claim 1, wherein the rapid cooling of the steel sheet to about 400* C. is effected by quenching the steel sheet.
 4. A process according to claim 2, wherein the rapid cooling of the steel sheet to about 400* C. is effected by quenching the steel sheet. 